1. Field of the Invention
The present invention relates to an LC composite part constituted by forming inductors and capacitors at an inside of a substrate including both a magnetic material and a dielectric material.
2. Discussion of the Background
Background EMI (Electro Magnetic Interference) filters have been used as a countermeasure for high frequency noise in an electronic device. Generally, an EMI filter is constituted by combining individual capacitor and inductor elements. Many types of these EMI filters have been proposed, such as L-type EMI filters constituted by combining one capacitor and one inductor and .pi.-type filters constituted by combining two capacitors and one inductor.
When an EMI filter is constituted by combining capacitor and inductor elements in this way, a total size of the EMI filter is magnified, a wide area is needed in attaching the EMI filter to a circuit board or the like, and a greater number of steps are required to combine the individual elements.
In recent years, there have been proposed filters which resolve the aforementioned problems. These filters include incorporating capacitors and inductors in a single chip (please see Japanese Unexamined Patent Publication No. JP-A-8-65080 and Japanese Unexamined Patent Publication No. JP-A-8-148381).
According to these publications, there is proposed a filter part having a chip-like shape where conductive films operating as inductors and capacitors are formed on a plurality of layers. Each of the layers include a mixture of a dielectric material and a magnetic material. In addition, the plurality of layers are laminated and sintered, thereby forming an LC filter of an L-type or a .pi.-type at an inside portion of the filter part.
All of the inductors formed at the inside of the filter part described in the publications have linear conductive films sandwiched by mixtures including a dielectric material and a magnetic material. Therefore, it is difficult to provide a large inductance in limited dimensions.
A large inductance may be formed with an inductor having a spiral structure extending over a plurality of layers. This can be accomplished by forming half-round or full-round conductive films on a single layer including a mixture of a dielectric and magnetic material. The conductive films are extended to a next layer via conductors in through holes. However, when such a spiral inductor structure is formed in the mixture substrate, the magnetic forces of the inductors adversely effect an influence on a capacitor contiguous to the inductor at the inside of the substrate. Therefore, the characteristic of the filter is significantly deteriorated.
Further, a multi-stage LC filter is preferably to create an LC filter circuit with excellent filter characteristic. However, when a multi-stage type LC filter using the L-type and .pi.-type filter proposed in the above-mentioned publications, the dimensions of the filter are excessively enlarged to provide a large inductance.
Further, when the mixture of the dielectric and magnetic material is used, the inductor has a considerably large residual capacitor that is equivalently connected to the inductor in parallel, because the dielectric material is included. Accordingly, an LC filter circuit with excellent characteristics is difficult to obtain.